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No door-to-door canvassing here: This census involved the direction of some 3 billion pings toward 2.8 million allocated Internet addresses from three machines over the course of two months.

And, oddly enough, the researchers say their project – designed to help predict future trends and improve ‘Net security -- received data-presentation advice from an unexpected source: a comic strip, albeit one fittingly made popular by the Web.

Such a comprehensive census of the “visible Internet” had not been undertaken since 1982 when David Smallberg conducted a full accounting of what was then a a mere 315 allocated addresses (RFC 832), according to researchers at the University of Southern California Information Sciences Institute.

Many (61 percent) of the pings received no response at all. Many others got a "do not disturb" or "no information available" response that many network administrators program into their routers and firewalls. Some of the non- replies were probably also due to firewalls intentionally blocking the pings. Still, as the census went on, millions of sites did respond, positively and negatively, and a unique Internet atlas took shape.

The research comes amidst an ongoing and somewhat controversial transition from IPv4 and its dwindling supply of Internet addresses to IPv6, which backers tout as having a virtually inexhaustible supply.

Presenting the census results graphically was a major challenge, one which researchers met through the help of a popular Web cartoon.

These addresses appear in the chart as a grid of squares, each square representing all the addresses beginning with the same first number ("128," in the preceding example). The map is arranged in not in simple ascending numerical order, but instead in a looping pattern called a Hilbert curve, which keeps adjacent addresses physically near each other, and also makes it possible to zoom seamlessly in to show greater detail. "The idea of using a Hilbert curve actually came from a web comic, xkcd," said Heidemann.

"Internet census data is useful for several reasons", Heidemannn says. "As the Internet use becomes widespread, we are running out of Internet addresses—good predictions by Geoff Huston suggest all addresses may be allocated as soon as early 2010. The IETF (Internet Engineering Task Force, the technical body that manages the Internet) has anticipated this since the 1990s and designed a new protocol, IPv6, to solve this problem, but deployment has been slow. Our data can help illustrate the need to move forward."

It's hoped that the census also can improve Internet security. In fact, the Department of Homeland Security "supported our work with the goal of improving network security," said Heidemann, pointing to the work of ISI researcher Jelena Mirkovic that is using this census data to study how worms spread in the Internet. Other researchers have plotted maps of where cyber-attacks originate.

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